[core] Allow pk-clustering-override without explicit DV for first-row merge engine

First-row merge engine already has built-in deletion vector semantics,
so requiring users to explicitly enable deletion-vectors is unnecessary.

Author: JingsongLi

docs/content/primary-key-table/pk-clustering-override.md

@@ -50,6 +50,23 @@ CREATE TABLE my_table (
 );
 ```
 
+For `first-row` merge engine, deletion vectors are already built-in, so you don't need to enable them explicitly:
+
+```sql
+CREATE TABLE my_table (
+    id BIGINT,
+    dt STRING,
+    city STRING,
+    amount DOUBLE,
+    PRIMARY KEY (id) NOT ENFORCED
+) WITH (
+    'pk-clustering-override' = 'true',
+    'clustering.columns' = 'city',
+    'merge-engine' = 'first-row',
+    'bucket' = '4'
+);
+```
+
 After this, data files within each bucket will be physically sorted by `city` instead of `id`. Queries like
 `SELECT * FROM my_table WHERE city = 'Beijing'` can skip irrelevant data files by checking their min/max statistics
 on the clustering column.
@@ -60,7 +77,7 @@ on the clustering column.
 |--------|-------------|
 | `pk-clustering-override` | `true` |
 | `clustering.columns` | Must be set (one or more non-primary-key columns) |
-| `deletion-vectors.enabled` | Must be `true` |
+| `deletion-vectors.enabled` | Must be `true` (not required for `first-row` merge engine) |
 | `merge-engine` | `deduplicate` (default) or `first-row` only |
 
 ## When to Use

paimon-core/src/main/java/org/apache/paimon/schema/SchemaValidation.java

@@ -522,7 +522,7 @@ private static void validateForDeletionVectors(CoreOptions options) {
                         || options.changelogProducer() == ChangelogProducer.LOOKUP,
                 "Deletion vectors mode is only supported for NONE/INPUT/LOOKUP changelog producer now.");
 
-        // pk-clustering-override mode requires deletion vectors even for first-row
+        // pk-clustering-override mode allows deletion vectors for first-row
         if (!options.pkClusteringOverride()) {
             checkArgument(
                     !options.mergeEngine().equals(MergeEngine.FIRST_ROW),
@@ -847,7 +847,8 @@ public static void validatePkClusteringOverride(CoreOptions options) {
                 throw new IllegalArgumentException(
                         "Cannot support 'pk-clustering-override' mode without 'clustering.columns'.");
             }
-            if (!options.deletionVectorsEnabled()) {
+            if (!options.deletionVectorsEnabled()
+                    && options.mergeEngine() != CoreOptions.MergeEngine.FIRST_ROW) {
                 throw new UnsupportedOperationException(
                         "Cannot support deletion-vectors disabled in 'pk-clustering-override' mode.");
             }

paimon-core/src/test/java/org/apache/paimon/separated/ClusteringTableTest.java

@@ -519,6 +519,22 @@ public void testFirstRowBasic() throws Exception {
                 .containsExactlyInAnyOrder(GenericRow.of(1, 100), GenericRow.of(2, 200));
     }
 
+    /** Test first-row mode without explicit deletion-vectors enabled. */
+    @Test
+    public void testFirstRowWithoutDeletionVectors() throws Exception {
+        Table firstRowTable = createFirstRowTableWithoutDv();
+
+        // Write initial data
+        writeRows(firstRowTable, Arrays.asList(GenericRow.of(1, 100), GenericRow.of(2, 200)));
+
+        // Write same keys with different values - should be ignored (first-row keeps first)
+        writeRows(firstRowTable, Arrays.asList(GenericRow.of(1, 999), GenericRow.of(2, 888)));
+
+        // Should still see the first values
+        assertThat(readRows(firstRowTable))
+                .containsExactlyInAnyOrder(GenericRow.of(1, 100), GenericRow.of(2, 200));
+    }
+
     /** Test first-row mode with multiple commits. */
     @Test
     public void testFirstRowMultipleCommits() throws Exception {
@@ -915,6 +931,22 @@ private Table createFirstRowTable() throws Exception {
         return catalog.getTable(identifier);
     }
 
+    private Table createFirstRowTableWithoutDv() throws Exception {
+        Identifier identifier = Identifier.create("default", "first_row_no_dv_table");
+        Schema schema =
+                Schema.newBuilder()
+                        .column("a", DataTypes.INT())
+                        .column("b", DataTypes.INT())
+                        .primaryKey("a")
+                        .option(BUCKET.key(), "1")
+                        .option(CLUSTERING_COLUMNS.key(), "b")
+                        .option(PK_CLUSTERING_OVERRIDE.key(), "true")
+                        .option(MERGE_ENGINE.key(), "first-row")
+                        .build();
+        catalog.createTable(identifier, schema, false);
+        return catalog.getTable(identifier);
+    }
+
     private void writeRows(List<GenericRow> rows) throws Exception {
         writeRows(table, rows);
     }